1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device and, more specifically, to a manufacturing method of a semiconductor device having a step of forming a contact plug by etching back a refractory metal layer that is formed on the entire interlayer insulating film having a connection hole.
2. Description of the Related Art
In recent years, with increases in the integration density of semiconductor devices and their performance as exemplified by those in VLSIs (very large scale integrated circuits) and ULSIs (ultra large scale integrated circuits), the proportion of the area occupied by wiring portions to the entire area of a device chip now tends to increase. The multilayered wiring technique is now indispensable for implementing semiconductor devices without causing marked increases in chip area. Conventionally, it is a common wiring forming method to form a metal thin film of, for instance, aluminum by sputtering. However, in circumstances that surface level differences of a substrate and the aspect ratios of connection holes are increased due to the advancement of the multilayered wiring, insufficient step coverage of sputtering may cause connection failures between an upper-layer interconnection and a substrate and between interconnections, which is one of the major problems of the above method.
In view of the above, a technique of filling in a connection hole of a large aspect ratio by growing selectively, i.e., in the connection hole, a refractory metal such as tungsten, molybdenum, or tantalum or some other metal such as aluminum or copper has been proposed recently. A typical means for such selective growth is a selective CVD (chemical vapor deposition) method in which a metal is deposited by reducing a gas of a metal fluoride, an organometallic compound, or the like with a lower-layer wiring material. However, although the selective CVD method has provided superior results in the research level, its introduction to mass-production is difficult because of several problems. For example, the selectivity gradually decreases during continuous processing, and the controllability in removing excessive growth portions called nailheads by etch back is low. A blanket CVD method in which a metal or an alloy is deposited on the entire substrate now attracts much attention as an alternative to the selective CVD method. In a typical process of the blanket CVD method, a layer of a refractory metal such as tungsten is formed so as to cover the entire insulating film that is formed with a connection hole and to fill in the connection hole.
Incidentally, etching back a refractory metal layer is necessary to fill in a connection hole with a refractory metal layer and use it as what is called a contact plug. In this etch back step, it is a common procedure to perform overetching of tens of percent in consideration of non-uniformity in processing rate in a wafer surface and among wafers. However, even in the same wafer, because of a variation in plasma density in an etching apparatus, a variation in the temperature profile of the wafer surface, and other factors, a rapid decrease of an etching area due to exposure of an interlayer insulating film occurs at an earlier stage in a region of a high etching rate than in other regions. The rapid decrease of an etching area in the region of a high etching rate causes a problem that etching species that are rendered relatively excessive due to absence of the refractory metal as bonding counterparts are concentrated in the connection hole and erode, to a large extent, the refractory metal layer and a barrier metal layer that are buried in the connection hole.
FIGS. 1A-1C are schematic sectional views of a wafer showing, in order of steps, a conventional process of forming a contact plug, and exemplify how a refractory metal layer 5 and a barrier metal layer 4 is eroded greatly in the blanket CVD method.
As shown in FIG. 1A, an interlayer insulating film 3 having a connection hole 6 for an impurity diffusion region 2 is formed on a substrate 1 that has been formed with the impurity diffusion layer 2 in advance. A barrier metal layer 4 having TiN/Ti layers is formed so as to cover the entire substrate 1 and not to fill in the connection hole 6. Further, a refractory metal layer 5 is formed by the blanket CVD method. Then, as shown in FIG. 1B, if the refractory metal layer 5 is etched back by using a fluorine-type gas, in a region of a high etching rate F* radicals are rendered excessive at an early stage after exposure of the surface of the barrier metal layer 4. The excess F*, radicals are concentrated adjacent to the surface of the refractory metal layer 5 that is buried in the connection hole 6, whereby a refractory metal eroded portion 7 is formed during overetching.
When the barrier metal layer 4 is further etched back under different conditions than in the case of FIG. 1B, radicals are rendered excessive at a time point when the surface of the interlayer insulating film 3 is exposed and the excess radicals are concentrated adjacent to a small cross-section of that portion of the barrier metal layer 4 which is buried in the connection hole 6. As a result, a barrier metal eroded portion 8 is formed as shown in FIG. 1C.
The phenomenon that the etching rate increases rapidly is generally called the loading effect. And the phenomenon that radicals are rendered relatively excessive as the area of an etching material layer decreases is a problem that frequently occurs in processes of forming a contact plug of a refractory metal layer by using the blanket CVD method and etching. There is a possibility that the loading effect will become more remarkable in the future in the field of semiconductor device manufacture because wafer diameters will increase with device chip sizes and single-wafer system plasma etching apparatuses that perform high-rate etching by using high-density plasma which does not cause a throughput reduction will become the mainstream. Therefore, it is now desired that an effective solution to the loading effect be established as early as possible.